Controller for a switched mode power supply

1. A controller for a switched mode power supply, the switched mode power supply comprising one or more windings, the controller comprising: a fixed speed timer; a threshold setter configured to a set a threshold for the timer in accordance with a peak value of a current through one of the one or more windings; a secondary stroke detector configured to start the fixed speed timer upon detection of the start of a secondary stroke of the switched mode power supply; a sampler configured to sample a voltage across one of the one or more windings when a count of the fixed speed timer reaches the threshold.

2. The controller of claim 1 , wherein the secondary stroke detector is configured to start the fixed speed timer in response to detection of the power switch of the switched mode power supply being turned off.

3. The controller of claim 1 further configured to: determine the length of the secondary stroke; determine a sampling moment as the instant that the sampler samples the voltage across the winding when the threshold is reached; wherein the threshold setter is configured to modify the threshold for use in a subsequent switching cycle of the switched mode power supply in accordance with the sampling moment and the length of the secondary stroke.

4. The controller of claim 3 , further configured to determine the length of the secondary stroke using one or more features of ringing in a voltage at the winding that follows a demagnetization period.

5. The controller of claim 4 , further configured to identify instants in time when a derivative of the voltage across the winding with respect to time is zero, and determine the end of the secondary stroke using the identified instants in time.

6. The controller of claim 4 , further configured to identify instants in time when the voltage at the winding crosses zero, and determine the end of the secondary stroke using the identified instants in time.

7. The controller of claim 3 , further configured to modify the threshold for use in a subsequent switching cycle in accordance with the ratio between the length of the secondary stroke and the time between start of secondary stroke and the sampling moment.

8. The controller of claim 3 , further configured to modify the threshold for use in a subsequent switching cycle in accordance with the difference between the length of the secondary stroke and the time between start of secondary stroke and the sampling moment.

9. The controller claim 1 , wherein the secondary stroke detector is configured to compare a signal representative of a voltage across the winding with a secondary stroke threshold voltage in order to detect the start of a demagnetization period.

10. The controller of claim 9 , wherein the secondary stroke threshold is about zero volts.

11. The controller of claim 1 further configured to control operation of a switch of the switched mode power supply in accordance with the sampled voltage.

12. The controller of claim 1 , wherein the switched mode power supply is a flyback converter having a transformer, the threshold setter is configured to a set the threshold for the timer in accordance with a peak value of a current through the primary winding, and the sampler is configured to sample a voltage across an auxiliary winding when the count of the fixed speed timer reaches the threshold.

13. A switched mode power supply comprising the controller of claim 1 .

14. A method of sampling a voltage across a winding in a switched mode power supply, the method comprising: setting a threshold in accordance with a peak value of a current through a winding of the switched mode power supply; detecting the start of a secondary stroke of the switched mode power supply; starting a fixed speed timer upon detection of the start of the secondary stroke; and sampling a voltage across the winding when a count of the timer reaches the threshold.

15. The method of claim 14 , further comprising controlling an operation of a switch of the switched mode power supply in accordance with the sampled voltage.